Search All of the Math Forum:

Views expressed in these public forums are not endorsed by NCTM or The Math Forum.

Notice: We are no longer accepting new posts, but the forums will continue to be readable.

Topic: Re: Applications of a simple differential equation:]
Replies: 0

 LnMcmullin@aol.com Posts: 10,730 Registered: 12/3/04
Re: Applications of a simple differential equation:]
Posted: Sep 21, 1997 6:15 PM

In a message dated 97-09-21 17:50:02 EDT,mark snyder wrote

<<
what that means is that e^x has no meaning if x has a dimension. This
comes from the power series for e^x: it has all powers of x in it, so if x
has dimensions of, say, feet, then e^x would have to have dimensions of ft
+ft^2+ft^3+..., which is clearly nonsensical (you can't add a length to an
area and get something which corresponds to any physical quantity: what
would be its dimension?). >>

Mark,
I beg to differ. In the power series x has all the powers as you say, but the
coefficients have the various derivatives which have their own units. These
should cancel the "extra" units of the powers of x. For example

s(x) in feet as a function of time in seconds
s'(x) in ft / sec times (x^1 sec) = feet
s''(x) in ft / sec/sec times (x^2 sec^2) = feet
s'''(x) in ft sec/ sec/ sec times (x^3 sec^3)= feet
etc.

Lin McMullin
Ballston Spa, NY